Question

Two simple pendulum, whose lengths are 100cm and 121cm, are suspended side by side. Their bobs are pulled together and then released. After how many minimum oscillations of the longer pendulum, will the two be in phase again?

(a) 11

(b) 10

(c) 21

(d) 20

The question was posed to me during an internship interview.

My doubt is from Wave Motion topic in section Waves of Engineering Physics I

Answer 1

Correct choice is (b) 10

The explanation: T∝√l

For the two pendulums in same phase

nT1=(n+1) T2

n√(l1)=(n+1) √(l2)

n√121=(n+1)√100

n×11=(n+1)10

n=10.